This invention relates to a membrane-electrode assembly (MEA) and, more particularly, to MEAs containing partially reduced metal oxide anode porous electrode structures for use with liquid or vapor methanol feed fuel cells in conjunction with proton-exchange membrane (PEM) solid electrolytes.
A fuel cell is a device which converts the energy of a chemical reaction into electricity. It differs from a battery in that the fuel and oxidant are stored external to the cell, which can generate power as long as the fuel and oxidant are supplied. The present invention relates to fuel cells in which the fuel is a liquid or vapor methanol/water mixture and the oxidant is air or oxygen. Protons are formed by oxidation of methanol at the anode and pass through a solid ionomer proton-exchange membrane electrolyte from anode to cathode. Electrons produced at the anode in the oxidation reaction flow in the external circuit to the cathode, driven by the difference in electric potential between the anode and cathode and can therefore do useful work.
The electrochemical reactions occurring in a direct methanol fuel cell which contains an acid electrolyte are:
Anode CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +6H.sup.+ +6e.sup.- (1) PA0 Cathode 3/2O.sub.2 +6H.sup.+ +6e.sup.- .fwdarw.3H.sub.2 O (2) PA0 Overall CH.sub.3 OH+3/2O.sub.2 .fwdarw.CO.sub.2 +2H.sub.2 O (3)
Many catalysts to promote methanol oxidation (Reaction 1) have been evaluated in the prior art due to the high polarization of this reaction on Pt. The types of catalysts investigated include: (1) noble metals, (2) noble metal alloys, (3) alloys of noble metals with non-noble metals, (4) chemisorbed layers on Pt, (5) platinum with inorganic material, and (6) redox catalysts. Based on literature reports, Pt--Ru appears to be the best methanol-oxidation catalyst in acidic electrolytes.
As shown by Vielstich, Kuver and Krausa in Proceedings of the Symposium on Batteries and Fuel Cells for Stationary and Electric Vehicle Applications, published by The Electrochemical Society, Pennington, N.J., Vol. 93-8, p. 269, 1993, the increased activity of Pt--Ru, or more specifically, (Pt--Ru)O.sub.x, over pure Pt for methanol oxidation is speculated to be due to a bi-functional mechanism, in which methanol is selectively adsorbed onto Pt atoms and OH.sup.- is chemisorbed on RuO.sub.x, providing the oxygen necessary for oxidation of the adsorbed methanol species. Alternatively, as shown by Hamnett and Kennedy in Electrochimica Acta, Volume 33, p. 1613, 1988, the RuO.sub.x may promote formation of a Pt oxide at a nearby site and this Pt oxide reacts with the adsorbed methanol species. In either case, Ru must be oxided to promote the methanol-oxidation reaction on Pt. Also, to achieve maximum performance, it is very desirable to have an electrochemically clean (Pt--Ru)O.sub.x surface, free of adsorbing anions that may be used as electrolytes such as sulfate ions. The perfluorosulfonic acid ionomer that coats the noble metal oxide particle of this invention does not adsorb onto the active surface and locally provides rapid proton transport.
The methanol/water feed to a direct methanol fuel cell (DMFC), and more particularly to a proton-exchange membrane fuel cell (PEMFC) may be in liquid as well as the vapor phase. The PEMFC uses a hydrated sheet of a solid ionomer perfluorinated ion-exchange membrane as a solid electrolyte in the fuel cell; catalytic electrodes are intimately bonded to each side of the membrane. These membranes are commercially available from either DuPont (under the tradename Nafion) or from Dow Chemical. From a systems standpoint, operation on liquid methanol/water containing some of the corresponding vapor appears to be more advantageous. As shown by Cameron, Hards, Harrison and Potter in Platinum Metals Review, 31, 131, 1987, liquid feed stream operation of a DMFC which utilizes a membrane required a combination electrolyte system, consisting of the PEM and an additional acid, generally H.sub.2 SO.sub.4. The H.sub.2 SO.sub.4 is added to the fuel stream to provide ionic conductivity throughout the anode structure, which otherwise is limited to only the catalyst in direct contact with the solid membrane. The H.sub.2 SO.sub.4 penetrates the anode structure, providing ionic conductivity throughout the electrode, thus allowing most of the catalyst to be utilized, resulting in improved performance. However, use of H.sub.2 SO.sub.4 is undesirable due to sulfate species adsorbing onto the electrode surface, sulfuric acid's corrosive nature and the possibility of shunt current formation within bipolar fuel cell stacks.
These problems are addressed by the present invention, an MEA comprised of a partially reduced platinum-ruthenium oxide, (Pt--Ru)O.sub.x, anode porous electrode structure which provides rapid proton conductivity and activity throughout the electrode structure with a PEM solid electrolyte and a cathode gas-diffusion electrode, with the anode porous electrode structure and cathode gas-diffusion electrode in intimate contact with the PEM.
Accordingly, it is a principal object of the present invention to provide a PEM fuel cell with a partially reduced catalyst, (Pt--Ru)O.sub.x, capable of operation on a direct liquid or vapor methanol/water mixture.
Another object of the present invention is to provide an anode porous electrode structure capable of operation on direct methanol/water without addition of liquid supporting electrolyte.
Still another object is to provide uniform continuity of electronic and ionic paths about all of the catalyst sites.
Still another object is to provide an electrochemically clean reduced (Pt--Ru)O.sub.x particle surface by use of a coating with a perfluorosulfonic acid film and synergistically promote the direct methanol oxidation reaction by rapid proton transport.